DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following examples illustrate the invention.

Example 1

2 kg Al.sub.2 O.sub.3 C produced pyrogenically from aluminum trichloride in an oxyhydrogen flame and having the properties given above are placed in a 135 liter Lodige mixer and sprayed with 100 g of a silane mixture consisting of 50 g trimethoxyoctylsilane and 50 g 3-aminopropyltriethoxysilane with the mixer running. The mixture is mixed for 15 minutes more. The silanized oxide is tempered 2 hours at 120

Physico-chemical properties of the surface-modified aluminum oxide

______________________________________Carrier                  Al.sub.2 O.sub.3 CSurface          (m.sup.2 /g)                    92Stamping density (g/l)   70Drying loss      (%)     0.9Annealing loss   (%)     5.3C content        (%)     2.9pH                       5.7______________________________________

Example 2

The aluminum oxide waterproofed according to Example 1 is tested in a positive toner system. The toner system consists of the following components:

______________________________________Pigment black Printex 35  7%Copy-Blau PR (Hoechst AG) 3%Toner resin               90%______________________________________

The repeated activation was tested with this toner and a high charge stability in comparison to the raw toner was determined (see FIG. 1).

Copy-Blau PR is a charge regulating agent for positive toners. It is characterized as follows:

Area of application:

1. Charge regulating agents for positive toners (1- or 2-component toners for copiers or laser printers)

2. Clearing agents for black toners

______________________________________Chemical characterization:          triphenylmethane derivativeThermal resistance:          >200Solubility:    insoluble in water          slightly soluble in organic solvents______________________________________

The toner resin used is characterized as follows:

______________________________________        Unit       Theoretical value______________________________________Melt flow Index.sup.1)          g/10 min      5-10(150Viscosity number.sup.2)          cm.sup.3 /g  37-43Weight loss.sup.3)          % by weight  <1Residual monomers.sup.4)          % by weight  <0.35Styrene                     <0.25n-BMA                       <0.10Other product properties:Monomer composition          70% by weight          styrene          30% by weight          n-butyl-          methacrylateGlass transition          60-65temperature Tg.sup.5)Average grain diameter.sup.6)          0.200-0.314 mm(d 50% RS)______________________________________ .sup.1) DIN 53 735, 2/88 edition Specimen pretreatment: Drying at 50 4 hours drying oven, 50 .sup.2) DIN 7745, 1/80 edition .sup.3) IR drier until weight constancy .sup.4) Gas chromatography .sup.5) DSC method, ASTM D 3418/75 .sup.6) DIN 53 734, 1/73 edition, evaluation according to DIN 66 141, 2/7 edition

__________________________________________________________________________The pigment black Printex 35 is characterized as follows:RCF (regular color furnace)Density: (g/cm.sup.3) 1.8-1.9Product specifications__________________________________________________________________________                  DBP Adsorption    Extract      Depth of           Color  (mg/100 g)                           Volatile contents                                         SievePrintex    Color           Strength                  powder beads                           Components                                    toluene                                         Residue35 RCF  Class      M.sub.γ -value           IRB 3 = 100                  Powder                       Beads                           (%)    pH                                    (%)  (%)__________________________________________________________________________Furnace Blacks   RCF      236  100    42   42  0.9    9.5                                    <0.1 0.05Printex 35Further technical data             Stamping Density    Ashing Residue             Powder                   Beads                        Particle Size (nm)                                  BET Surface (m.sup.2 /g)__________________________________________________________________________Furnace Blacks    0.3      420   550  31        65Printex 35__________________________________________________________________________

The q/m measurement takes place under the following conditions:

______________________________________98% carrier (spherical ferrite (80-100 m))2% aluminum oxide according to Example 15 Activation:        Rolling fixture, 360 rpms in 40 ml        glass bottle, weighed portion 40 g,        developer______________________________________

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a graph plotting charge per unit mass as a function of activation time for raw and treated toner.

The present invention relates to a surface-modified, pyrogenically produced aluminum oxide.

BACKGROUND OF THE INVENTION

It is known to use powdery toners containing pyrogenically produced surface-modified silicon dioxide in electrostatic developing processes. Various silanes, especially dimethyldichlorosilane are used for surface modification (See U.S. Pat. No. 3,720,617).

It is also known that pyrogenically produced silicon dioxide waterproofed with compounds of the general formula ##STR1## can be added to positively chargeable resin powders in order to increase their flowability (See published European Patent Application EP-A 0,293,009).

Published German Patent Application DE-A 12 09 427 discloses aluminum oxide whose surface has been modified with halogen silanes can be added to electrographic developing powders.

A similarly treated aluminum oxide is described in Published Japanese Patent Application JP-OS 31442 (Nippon Aerosil Corporation).

The known method has the disadvantage that it must use an organic solvent system. Alcohols, hydrocarbons and halogenated hydrocarbons are used as solvents which cannot be completely removed from the reaction product.

SUMMARY OF THE INVENTION

The object of the present invention is to avoid these problems and produce a solvent-free, waterproofed aluminum oxide.

The present invention provides a surface-modified, pyrogenically produced aluminum oxide which is surface modified with a silane mixture consisting of silane A (trimethoxyoctylsilane) and silane B (3-aminopropyltriethoxysilane) having the chemical formulas: ##STR2## The surface-modified, pyrogenically produced aluminum oxide has the following physico-chemical properties:

______________________________________Surface        (m.sup.2 /g)  50 to 100Stamping density          (g/l)         50 to 90Drying loss    (%)           <5Annealing loss (%)           5.0 to 15C content      (%)           0.5 to 12pH                           4 to 8.______________________________________

The present invention also provides a method of producing the surface-modified, pyrogenically produced aluminum oxide in which the pyrogenically produced aluminum oxide is placed in a mixer and sprayed, with the mixer running, with the mixture of silane A and silane B. The silane and aluminum oxide are mixed after the addition of the silane mixture and the resulting mixture is tempered at 100 150

The ratio of aluminum oxide to silane mixture can be 0.5 to 40 parts by weight silane mixture per 100 parts by weight aluminum oxide.

The silane mixture can consist of 1 to 99 parts by weight silane A and 99 to 1 parts by weight silane B.

A mixture can be used with preference consisting of 50.+-.20 parts by weight silane A and 50.+-.20 parts by weight silane B.

A particularly suitable aluminum oxide is Aluminum Oxide C which is produced pyrogenically from aluminum trichloride by flame hydrolysis in an oxyhydrogen flame and which has the following physico-chemical characteristics:

______________________________________Al.sub.2 O.sub.3 C______________________________________AppearanceSurface according to BET                m.sup.2 /g                          100 .+-. 15Average size of the primary particles                nanometer 20Stamping density.sup.1)                g/l       --Drying loss.sup.2)   %         <5(2 hours at 105Annealing loss.sup.2)6)                %         <3(2 hours at 1000PH.sup.3)                      4-5(in 4% aqueous dispersion)SiO.sub.2.sup.5)     %         <0.1Al.sub.2 O.sub.3.sup.5)                %         >99.6Fe.sub.2 O.sub.3.sup.5)                %         <0.02TiO.sub.2.sup.5)     %         <0.1HCl.sup.5)7)         %         <0.5Sieve residue.sup.4) %         <0.05according toMocker (45 m)packing drum size (net)                kg        5normal goodscompressed goods     kg(additive "V)______________________________________ Technical data of the AEROSIL standard types .sup.1) according to DIN 53 194 .sup.2) according to DIN 55 921 .sup.3) according to DIN 53 200 .sup.4) according to DIN 53 580 .sup.5) relative to the substance annealed 2 hours at 1000 .sup.6) relative to the substance dried 2 hours at 105 .sup.7) HCl content is a component of the annealing loss

The waterproofed aluminum oxide of the invention has the advantage that it has no solvent components. It can be used in toners for copiers.

This is a division of application Ser. No. 08/141,083, filed Oct. 27, 1993 now U.S. Pat. No. 5,419,928 which is a Divisional application of Ser. No. 08/012,163 filed Jan. 28, 1993 now U.S. Pat. No. 5,384,194.